1. Field of the Invention
The present invention relates to a magnetooptical information recording apparatus in which a magnetooptical recording medium is irradiated with a light beam and a bias magnetic field is applied to the thus irradiated portion thereby recording information on said medium or erasing information recorded on said medium, and in particular such apparatus employing an electromagnet as the means for applying said bias magnetic field.
2. Related Background Art
The magnetooptical information recording apparatus has been the target of active development works in recent years as an erasable-rewritable memory of large capacity. The magnetooptical recording mediums employable in such apparatus are mostly disk-shaped ones, or magnetooptical disks. The magnetooptical disk is composed of a vertically magnetizable layer formed on a substrate such as glass or plastic, and said vertically magnetizable layer is magnetized in one direction. For recording information, the vertically magnetizable layer is irradiated with a laser beam digitally modulated according to an information signal, thus bringing the temperature of said layer beyond the Curie point thereof. Thus, a portion irradiated with the laser beam loses magnetization, and is magnetized again, when it is cooled, in a direction opposite to the direction of magnetization in the surrounding areas, by means of an externally applied DC bias magnetic field. In this manner a train of information pits is obtained corresponding to the information.
For reading the information recorded on the magnetooptical disk, the vertically magnetizable layer is irradiated with a reading laser beam, and the reading is achieved by means of magnetic Kerr effect, or a phenomenon that the direction of polarization of the reflected beam is varied according to the magnetizing direction of the vertically magnetizable layer. On the other hand, a train of recording pits can be erased by tracking and irradiating said pits with a laser beam and applying a DC bias magnetic field of a direction opposite to the magnetizing direction of pits, thereby rendering the direction of magnetization of said layer uniform again.
The direction of said DC bias magnetic field in erasing has naturally to be opposite to that in recording. In general, a DC bias magnetic field can be generated by a permanent magnet or an electromagnet, but the use of the latter is common since the former requires a specific mechanism and a time for inverting the magnetic field.
FIG. 1 is a perspective view of a conventional magnetooptical information recording apparatus utilizing an electromagnet.
In FIG. 1, bias magnetic field applying means 21 is composed of a main yoke 22, side yokes 23, 24 positioned on both sides thereof, a yoke plate 26 connecting an end of the main yoke 22 and the side yokes 23, 24, and a coil 25 wound on the main yoke 22. The main yoke 22 is T-shaped in order to facilitate coil winding.
Said bias magnetic field applying means 21 is positioned opposite to an optical head 27 across a magnetooptical disk 28, and in such a manner that the longitudinal direction of the main yoke 22 lies in the radial direction of the magnetooptical disk 28. The optical head 27 is provided with a light source such as a semiconductor laser, an objective lens for converging the light from said light source onto the magnetooptical disk 28 etc., and is rendered movable, by an unrepresented mechanism, in the radial direction of the magnetooptical disk.
However, in the above-explained structure, the magnetic flux from the main yoke 22 enters the side yokes 23, 24 through the air, so that only a small portion of said magnetic flux can be concentrated on the information recording portion of the magnetooptical disk 28. Consequently, in order to obtain a magnetic field of a desired intensity, it has been necessary to employ a very strong current or a coil of several hundred turns.
Such strong current requires a large and , expensive power source, and results in an nonnegligible rise in temperature. On the other hand, an increased number of turns of the coil inevitably increases the dimension of the magnetic field applying device, and gives rise to a proportionally increased reactance of the coil, thus requiring a longer switching time from information recording to erasing or vice versa.